In recent years, 3GPP (3rd Generation Partnership Projects) has specified a mobile communication system called a “W-CDMA (Wideband-Code Division Multiple Access) system (or UMTS: Universal Mobile Telecommunications System)” based on CDMA (Code Division Multiple Access) techniques.
FIG. 1 shows a configuration example of the W-CDMA system. As shown in FIG. 1, the W-CDMA system includes a mobile station UE (User Equipment), a radio base station NodeB, a radio control station RNC (Radio Network Controller), and an exchange MSC/SGSN (Mobile Switching Center/Serving GPRS Support Node), thereby configuring a cellular mobile communication system.
Radio waves transmitted from the radio base station NodeB form an area called a “Cell” which serves as a control unit in the cellular system.
In the W-CDMA system, a cell is identified by a combination of a frequency band of a pilot channel and a spreading code called a “Primary Scrambling Code.” Usually, a pilot channel is linked with a cell on a one-to-one basis.
Here, the W-CDMA system can employ a configuration in which cells covering a geographically identical area but using different frequency bands for transmission of pilot channels overlay each other.
In the example of FIG. 1, a cell 01 and a cell 02 cover a geographically identical area while a cell 03 and a cell 04 cover another geographically identical area. Moreover, a frequency band 01 is allocated to the cell 01 and the cell 03 while a frequency band 02 is allocated to the cell 02 and the cell 04.
Here, a communication capacity is limited with a single frequency band since a bandwidth of a frequency band is defined. However, overlaying the cells to which the different frequency bands are allocated in the above described manner allows expansion of a communication capacity at the same spot.
The following methods are devised as communication setting methods in the configuration in which multiple cells employing different frequency bands overlay each other. One of the methods is a method of causing a mobile station UE which is in a standby mode in any of cells to set up communication only in the cell in which the mobile station UE is in the standby mode at the time of starting communication. The other one of the methods is a method of causing a mobile station UE to set up communication in a cell employing a different frequency band from that of a cell in which the mobile station UE is in the standby mode so as to achieve flexible load balancing.
In the latter case, communication qualities of the respective overlaid cells are not always the same. For this reason, it is usual to measure communication quality of a cell employing different frequency band by some means and to judge in advance whether or not it is possible to set up communication in the cell employing the different frequency band.
Here, the “communication quality” includes an electric field intensity of a pilot channel, a ratio (SIR: Signal To Interference Ratio) of an electric field intensity of a desired wave to an electric field intensity of an interference wave, an electric field loss, and the like.
When the communication quality is determined to be sufficient by a threshold judgment using these communication quality factors, the mobile station UE performs communication setting even in the cell employing the frequency band separate from that of the cell in which the mobile station UE is in the standby mode. If not so determined, the mobile station UE does not perform communication setting in the cell employing the frequency band separate from that of the cell in which the mobile station UE is in the standby mode.
Meanwhile, as means for acquiring the communication quality of the cell employing the frequency band separate from that of the cell in which a mobile station UE is in the standby mode (a standby cell), there is known a method of causing the mobile station UE to measure the communication quality of the cell employing the different frequency band during the mobile station UE is in the standby mode in advance and to notify the radio control station RNC of a measurement result with the measurement result included in a communication start request signal.
Meanwhile, “JP-A No. 2003-219449” discloses the following procedure in which a mobile station UE which is in a standby mode in a cell does not measure communication quality of a cell employing a different frequency band. In the procedure, a radio control station RNC executes cell selection processing in the course of a communication setting procedure on the basis of the communication quality of the cell in which the mobile station UE is in the standby mode. When it is not possible to select the standby cell, the radio control station RNC requests the mobile station UE to measure communication quality of a cell employing a separate frequency band and thus acquires the communication quality of the cell employing the separate frequency band.
In this procedure, the mobile station UE does not measure the communication quality of the cell employing the separate frequency band in the standby mode but executes the measurement of the communication quality of the cell employing the separate frequency band only when it is not possible to select the standby cell. This allows the mobile station UE in the standby mode to reduce the power consumption thereof.
In the meantime, communication services provided respectively in such overlaying cells may be different from each other because of reasons of installation and construction of the provider, such as costs and installation of equipment.
Examples of such communication services include an “HSDPA (High-Speed Downlink Packet Access) service”, an “HSUPA (High-Speed Uplink Packet Access) or EUL (Enhanced Uplink) service”, and the like, which have been progressively introduced.
These are the communication services for an uplink and a downlink included in a new version of 3GPP standards such as “Release 5” or “Release 6”, which can perform high-throughput and high-efficiency communication as compared to a conventional “Release 99 (for uplink and downlink) service”.
In the example of FIG. 1, an EUL service is provided only in the cell 02 but not in the cell 01.
In that case, even if a mobile station is an “EUL support mobile station” provided with a communication capability for the EUL service, when the mobile station is to set up communication in the cell 01, communication for a low-speed Release 99 (uplink) service is set up instead of communication for the EUL service. Only when the mobile station is to set up the communication in the cell 02, the communication for the EUL service is allowed to be set up.
Assume a case of employing the above-described form of providing the EUL service. According to the method disclosed in “JP-A No. 2003-219449”, the cell selection is executed based on the communication quality. Only when it is not possible to select the standby cell, the communication quality of the cell employing the separate frequency band is measured or the cell employing the separate frequency band is selected. Therefore, the EUL support mobile station which is in the standby mode in the cell 01 does not perform measurement of the communication quality in the cell 02 or cell selection of the cell 02 as far as the communication quality in the cell 01 is sufficient, but sets up the communication for the Release 99 (uplink) service in the cell 01 at the time of starting the communication.
Alternatively, instead of the method disclosed in “JP-A No. 2003-219449”, a method of setting up the communication uniformly in the cell 02 is conceivable in order to set up the communication for the EUL service. However, in that case, because of non-execution of measurement on the communication quality in the cell 02, the communication may be performed with poor communication quality.
As described previously, according to the conventional technique, the following problems are involved in a structure in which a cell A that provides a particular service (a particular communication service) and a cell B that does not provide the particular service overlay each other in different frequency bands. One of the problems is that the particular service is less frequently provided due to occurrence of a case where setting up communication for the particular service is failed. Another one of the problems is that if the communication for the particular service is set uniformly in the cell A in order to prevent the reduction in frequency, the communication may be performed at poor communication quality, thereby arising a concern for degradation in a service performance.